The present invention relates to a display device for displaying an image on a display panel illuminated by a back light source, and more particularly, to a display device which can adjust a voltage applied to a common electrode.
In a liquid crystal element used for a display device, a continuously applied DC voltage results in a degradation in image quality similar to burn-in. For this reason, in a liquid crystal display, a liquid crystal element is applied with signal voltages which include a positive voltage (Vsig+) and a negative voltage (Vsig−) alternately applied in each frame period. Also, when a voltage (VCOM) is applied to a common electrode of the liquid crystal element, the display luminance is determined in accordance with (Vsig+)−(VCOM) when the positive voltage is applied and with (VCOM)−(Vsig−) when the negative voltage is applied.
In this event, if an inappropriate voltage (VCOM) is applied to the common electrode, the display luminance differs when the positive voltage is applied to the liquid crystal element and when the negative voltage is applied to the liquid crystal element, resulting in flicker at a cycle of two frame periods. For reducing this flicker, it is necessary to accurately adjust the voltage VCOM. In addition, variations in characteristics introduced in liquid crystal panel manufacturing processes can cause an optimal VCOM voltage to differ from one liquid crystal panel to another. For adjusting this VCOM voltage to an appropriate voltage, a pattern for adjustment must be displayed to individually adjust each display device while measuring an actual display luminance visually or using a measuring instrument.
Since an increase in cost would be caused by this VCOM adjusting operation manually performed in the foregoing manner before shipment of liquid crystal display devices, JP-A-6-138842, JP-A-10-246879 describe automatic adjusting methods.
JP-A-6-138842 discloses a measurement of the brightness of an image projected by a liquid crystal projector using a photodiode disposed on a screen. A current flowing through the photodiode fluctuates depending on flicker. A voltage VCOM to a common electrode is adjusted by an electronic volume or the like while measuring the amount of fluctuations in the current. The flicker is reduced by adjusting the voltage VCOM so as to minimize the amount of fluctuations in the photodiode current.
JP-A-10-246879 discloses a liquid crystal panel which comprises a dummy pixel area outside of a display area of the liquid crystal panel, and a photodiode disposed on the dummy pixel area, as opposed to the display device of JP-A-6-138842 which has the photodiode disposed on the screen. This liquid crystal panel features in the photodiode disposed on the dummy pixel area, which does not prevent a display of an input video signal or require a screen. JP-A-10-246879 also describes that the flicker will prominently appear when a halftone checkered pattern is displayed on the dummy pixel area. Also, when an automatic VCOM adjustment function is incorporated in a liquid crystal display, the VCOM voltage can be adjusted to adapt to a variety of environments, for example, variations in temperature and the like.